Intelligence services throughout the Middle East and Europe are scrambling to track down more than two dozen fighters linked to al-Qaida who have recently left their base in southern Lebanon.

The missing men are thought to have gone to Europe by a newly established route through Syria, Turkey and the Balkans, and multiple intelligence sources in Lebanon warn that the group appears to be operational and could be planning attacks in Europe in the holiday season.

"Yes, they have left the camp," confirmed Munir al-Maqda, a senior Palestine Liberation Organisation official in the Ain el-Hilweh refugee camp, where the fighters had been hiding for the past few years. Because the Lebanese army is not allowed to enter the country's 13 Palestinian refugee camps, militants have long sought refuge inside them.

Two Lebanese intelligence service officials – who could only discuss the matter off the record – said Lebanon was co-operating with European intelligence organisations to track down the militants, who are described as "extremely dangerous".

The militants, who based themselves in an area on the outskirts of the Ain el-Hilweh camp – Lebanon's largest and most densely populated refugee centre, with more than 50,000 people living within one square mile – had been a disparate group of freelance fighters and jihadists thought to have carried out a series of attacks in Lebanon over the last five years that targeted the Lebanese army, Christian districts and UN peacekeepers in southern Lebanon.

Stephen Tankel, of the Carnegie Endowment for International Peace, has done field research in the Ain el-Hilweh camp and describes the band of militants as potentially dangerous because of their lack of overall control. This means that the group are receptive to outside direction, and leaves them open to new ideas and strategies, such as becoming operational in Europe as their options in Lebanon dwindle, Tankel said.

"They may not have been super-organised, but that does not mean they are not potentially dangerous," he said. "First, a number of them fought in Iraq, where they will have gained combat experience and, perhaps, some explosives expertise.

"Second, it's safe to assume that many of them are prepared to undertake 'martyrdom' operations. That said, they would benefit from guidance in terms of targets [choosing and surveying them] and logistical support in Europe."

In view of their experience in Iraq and overall "street credibility" among European jihadists, Tankel suggested that the group might be better able to operate successfully in Europe than in Lebanon, where they were closely monitored, not only by the intelligence services but by the Shia militant group Hezbollah. Hezbollah, despite sharing a dislike of western policies in support of Israel, is bitterly opposed to the al-Qaida ideology, which is linked to Sunni Muslims.

One European Union intelligence official confirmed to the Observer that an operation to hunt down Arab fighters recently arrived from Lebanon was under way, but could not link this group to recent concerns about possible holiday attacks by al-Qaida.

"We have received warnings of a significant militant plot in Europe during the holidays and we have been warned about these missing fighters from Lebanon," he said. "But we wish we knew if the two threats were related."